Black Carbon Reduces Snow Albedo

Snow reflects less solar radiation when contaminated with black carbon.

The Science

The authors developed an approach to isolate the effect of black carbon (BC) on snow albedo through laboratory experimentation with newly-developed processes for making both pristine and BC-laden snow and techniques for measuring the morphology, albedo, and BC content of this snow.

The Impact

The study verified that black carbon contamination at levels that have been found in natural settings appreciably reduces snow albedo contributing to global warming and near-worldwide melting of ice.

Summary

Climate models indicate that the reduction of surface albedo caused by black carbon (BC) contamination of snow contributes to global warming and near-worldwide melting of ice. However, model predictions of BC-caused snow albedo reduction over a range of BC levels and snow grain sizes have not been verified by measurements. The main reason is that the BC effect is typically masked in natural environments by other variables that influence albedo, such as snow grain size, snow density, snow depth and the interaction of sunlight with the underlying surface, tree cover and solar zenith angle. These methods enabled quantification of the snow albedo reduction associated with increasing amounts of BC and as a function of snow grain size. The study verified that black carbon contamination at levels that have been found in natural settings appreciably reduces snow albedo. Increasing the size of snow grains decreased snow albedo and amplified the radiative perturbation of black carbon, which justifies the aging-related positive feedbacks that are included in climate models. Moreover, these data provide an extensive verification of a snow, ice, and aerosol radiation model which will be included in the next assessment of the Intergovernmental Panel on Climate Change.